1. Field of the Invention
The present invention relates to an automatic document feeder provided on an image forming apparatus such as a copying machine.
2. Description of the Prior Art
To reduce a time for replacing original documents or to eliminate type procedure for the document replacement, various copying apparatuses provided with an automatic document feeder (hereinafter abbreviated as ADF) have been recently developed and commercially available. Such an ADF constitutes means for reducing indirect costs (time and labor). In contrast, to reduce direct costs (supplies, such as copy sheets and toner), several methods available are as follows: a duplex copying method, wherein two documents are copied to both sides of one copy sheet; and a method for reducing and copying two documents to one side of a copy sheet having the same size as each original document (two-document, single-face copying mode). In the former method, a pair of documents are unchangedly copied to one side of a copy sheet, while toner making for two copies is consumed. In copy sheet, while toner making for two copies is consumed. In contrast, the latter method is economical in that a copy sheet and toner each for only one sheet of document is consumed per two duplicates. Furthermore, when the above two methods combined, the result is extreme economization, since four documents are copied using both sides of one copy sheet.
Various conventional ADFs, however, feed documents one by one onto a platen. Accordingly, to perform two-document, one side copying, an operator is supposed to replace documents and place them on the platen per sheet of document, thus time and labor are not reduced.
To solve such disadvantages, improvements have been disclosed in the following patents, although all of them incur some problems. For example, an ADF for feeding two documents parallelly onto a platen has been disclosed in Japanese Patent Examined Publication No. 56-25657 (equivalent to U.S. Pat. No. 4,025,054). This type of ADF requires a time for parallelly setting documents on a document tray, and for rearranging the page order of documents after the documents are discharged. Additionally, this type of ADF also requires a space in the depth direction i.e. the direction perpendicular to the document feeding direction. As a result, the size of a copying apparatus is inevitably larger.
An ADF which is capable of serially placing two documents onto the platen by two transport belts disposed above the platen in tandem in the document feeding direction, has been disclosed in Japanese Patent Laid Open Publication Nos. 60-2942 and 60-84945. This type of ADF, however, also incurs a problem; the shadow between the two transport belts is unavoidably copied on a copy sheet when a thin document is copied.
Moreover, an ADF capable of serially setting two documents onto a platen has been disclosed in Japanese Patent Laid Open Publication No. 60-93462, wherein a second document separation device. This document feeding method is called the "serial double feeding". However, commercialization of a serial double feeding ADF is difficult due to its reliability, because accurate document feeding may not be achieved.
Additionally, the above-identified ADF which is capable of serially feeding two documents and placing them onto the platen in the document feeding direction involves a problem regarding document discharge from the platen. More specifically, since the two documents are placed without allowing an opening in between, the leading edge of the second document pushes the trailing edge of the first document, while two documents are discharged from the platen. Consequently, this may result in disturbance on the page order and alignment of documents discharged onto a discharge tray.